Though some of these methods do take some work up front, they can set you up to earn passive income for the long term.

Here are 21 ways to make money with minimum effort.

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1. Become a referral source for local businesses.

A lot of small, locally focused businesses — like landscapers, electricians, and home-improvement companies — spread by word of mouth. Many of them pay referral fees for people to help spread the good word about their business.

So the next time you have your plumber over, ask whether referrals are accepted. And if the plumber does a good job, make sure to take advantage of that and recommend the person to a friend.

From then on, whenever people buy your book, you get paid in royalties. The precise amount of royalties are contingent on your contract with the book publisher.

If you self-publish on Amazon or another self-publishing platform, there are standardized contracts that pay out depending on how many people download the e-book or read it through Amazon's e-book-lending program.

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3. Create an online course and get paid for it.

Let's say you're an expert in a subject but don't think a book is the right way to communicate the knowledge you want to share.

You can do it with an online course. You can make and upload one to Udemy, for example, and set your own price points. Whenever someone buys the course, you'll get paid.

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4. Make a website and get advertisement revenue.

If you're an expert in a subject that isn't fit for a book project or an online course, set up a website!

Use Wordpress, Weebly, or some other web-creation platform to make a site and make your guide there. Then sell online ads — which you can do with Google, for example— on your site so you get some spare revenue whenever someone reads your work.

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5. Or make a YouTube video guide.

Stick with me. Let's say you're an expert on a subject, but the best way to convey your information isn't in a book, an online course, or a post on the web. Instead, consider making a YouTube video.

As with the web, YouTube lets you set up ads on your videos. People searching for those subjects will find your videos, watch them, and then the ad revenue will roll in indefinitely.

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6. If you already have a website, use referral links.

Whether your website is an online guide, a blog, or whatever else, use referral links whenever you discuss a product.

Sites like Amazon, for example, let you add a bit of code in the URL of a product. If anyone buys the product through that URL, you get a percentage of the revenue from that purchase.

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7. Be in a commercial.

How much do you think he made for this commercial?
YouTube/Beats by Dre

Any actor will tell you that getting an acting gig — even a commercial — is tough. If you live in a major metropolitan area, though, it isn't as difficult to become an extra.

If the contract is good, you can make some serious money. One of my friends was featured in a Target commercial that was in heavy rotation during the 2012 Summer Olympics. He got $500 every time it aired and used the cash to pay off a good chunk of his college tuition.

8. Put your unused apartment or room on Airbnb.

If your kids have flown the coop and you have an extra room in your house, or if you're traveling for a while and won't be in your apartment, consider renting it out for the short term on websites like Airbnb or VRBO.

They basically allow you to turn your room into a hotel. You can list your room, set rental dates and prices, and people visiting the area will pay you to live in your room for the agreed-upon time.

How much you can make depends on the rental market in your area.

If you have an apartment in Williamsburg, Brooklyn, for example, you can stand to make a pretty penny by renting out your room while you go backpacking in Europe for the summer. But if you're in Pittsfield, Maine, you might not find as much demand for your house's extra bedroom.

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9. Sell old books on Amazon.

If you have a bunch of old books lying around that you never plan to read again, you can put them in a box and ship them to Amazon to sell for you. Whenever one gets sold, they'll take a cut and then give you the remainder of the profits.

11. Save your money in a high-yield savings account.

When you save your money in a savings account, banks often give you extra money based on interest.

Sometimes their interest rates can be low — below inflation rates, even — which means you're effectively losing money. The interest figure, often referred to as "APY," can differ based on which country or state you live in and what each bank offers in your area. On the low end, they're about 0.01%, assuming you meet the bank's minimum requirements.

But with a high-yield savings account, you can get an APY of about 1%. It isn't much, but it's significantly higher than what a low-interest account would offer.

Unless you have a lot of savings, it's hard to be eligible for a high-yield savings account from a traditional bank. Instead, you'll probably have to park your money in an online bank, like Ally or Synchrony. Since they don't have brick-and-mortar stores, they pass the money they save on rent to the consumers, with things like high-yield savings and by reimbursing users for ATM fees.

14. Get even more cash back with Ebates.

Thousands of online stores, whether J.Crew or Amazon, also offer cash back for purchases you make on their sites. You just need to know where to look.

The website Ebates catalogs almost all of them. If you shop with its browser extension installed, it'll tell you when cash back or coupons are available. You can also use the extension Wikibuy, which has Ebates integration built in, and additionally checks in the background if the item you're shopping for is cheaper on a different website.

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15. Sell your photography to the web.

Sites like INSIDER always need good photos for our content.

Sometimes we take our photos ourselves, sometimes we get them from news-wire services like Reuters and Getty, and sometimes we get them from stock image services, like Shutterstock and iStock.

For Shutterstock and iStock, anyone can submit their photos to be included in the service. You just have to follow their guidelines and upload your photos. Then when someone (like INSIDER) downloads your photos through them, you'll be paid a fee. If you're a photography hobbyist, it's a relatively simple way to make some money from your photographs.

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16. Get cash back on credit cards.

Credit cards often offer cash back as a reward for making purchases. This means that for every purchase you make the credit-card company will give you a small percentage of the money back.

The percentage differs between credit cards and categories. So, for example, one type of credit card might offer 1.5% cash back on all purchases. Another might offer 1% back on most purchases but 5% back on gas and travel purchases.

You also often have to reach a certain threshold of payment to be able to cash in. And cards will often allow you to redeem your cash back as "points" to purchase things directly from the credit-card company's website instead.

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17. Invest in real estate.

Michael Heiman / Getty

This one takes more cash upfront and more management, but the payout could be significant.

If you can invest in a real-estate venture, you can make a lot of rental money. Of course, there are a lot of obstacles to hurdle. Most important, you need a lot of capital to invest in the first place. And secondly, you need to make sure the place you're investing in will have a steady stream of rent-paying tenants.

But once you have a property that's established and has tenants, it's just a matter of upkeep.

If you invest in a venture with a full-time property manager, the job becomes much more passive. Renting out real estate is a considerably more simple and stable way to get extra cash flow every month with relatively little effort compared with other investment opportunities.

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18. Build an app.

If there's a function you want on your smartphone, try making an app for it. With sites like Code Academy, learning to code is easier than ever.

If your app is good and people like it, you can easily make money in two ways. One way is to charge for it in Apple's App Store or Android's Google Play Store. Or you can sell ads on it, as you would on a website.

19. Get paid for watching TV and playing video games.

It's the dream, isn't it?

Sites like Inboxdollars will pay you money to keep track of the television you watch, the video games you play, and other habits you might have already. All you have to do is take surveys to give your feedback, and they'll pay you.

The money comes from research firms that want to understand consumer habits.

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20. Get paid for browsing the web.

Similarly, Swagbucks will pay you for installing an add-on that tracks your searches. Like Inboxdollars, it's a middleman for research firms that want to understand how people use the web.

You earn points for doing everyday things and can redeem them for gift cards at stores you'd probably shop at anyway, like Amazon and Walmart.

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21. Get paid for improving products.

Companies that want to get consumer research will offer elaborate "missions" that you can sign up for on the app. They might involve going to a supermarket and checking the availability of granola bars or testing out a device.

While completing the mission, you need to answer survey questions, and they'll pay you for it. It's not always easy to get accepted to a mission, but if you do, you can stand to make much more money than with Inboxdollars or Swagbucks. It's not uncommon to see missions that pay out about $50.